Q: What can you tell me about the safety of milk thistle, assuming one uses a standardized, quality product? What is the proper dosage? And is there any reason to believe that it may prevent liver damage in people taking medications that have the possibility of causing liver damage?
A: Silybum marianum, called milk thistle because of the white veins that run through the leaves and produce a milky sap, has been used by herbalists and indigenous healers to prevent and treat liver disease for thousands of years. Research conducted since 1975, and especially during the last decade, has looked at the properties in milk thistle that might be responsible for protection of the liver and how this can be applied clinically.
Milk thistle is similar to medications used for hepatitis and cirrhosis and seems to have anti-inflammatory, antioxidant, and anti-fibrotic properties (fibrosis is the formation of scar tissue in the liver that occurs with cirrhosis); these functions may translate into help against viral and alcoholic liver disease.
To answer the first part of your question, studies suggest that milk thistle is safe for animals. Human studies conducted to date, however, have really been too small to render any conclusion about safety for people; very few side effects, though, from milk thistle have been reported. The occasional side effects have been mild allergic reactions and a laxative effect when milk thistle is taken in large doses. The latter tends to occur when people take doses much larger than generally recommended.
The usual dose is 200-400 milligrams of Silymarin, which is the name for the collection of the most active ingredients found in milk thistle. Standardized extracts of milk thistle should have 70-80% Silymarin; both the concentration and the amount of Silymarin should be marked on the product label. Milk thistle should be taken as a capsule.
Traditionally, milk thistle has been used to treat poisons found in the mushroom genus Amanita. More recently, researchers have been looking into whether Silymarin can protect against modern-day toxins and toxic effects, such as those that may occur with the improper use of acetaminophen (a common medication found in many over-the-counter preparations that can become toxic when combined with alcohol or if taken in very large quantities). In addition, milk thistle has been tested with anti-psychotic medications and seems to protect against liver damage from certain drugs used for psychiatric disorders such as schizophrenia.
Other than the examples mentioned, I am not aware of milk thistle being tested for protection against liver damage from other medications. This certainly seems possible, though, and you raise a very interesting research question for the future!